In recent years, recycling of water resources has been regarded as important, and wastewater has been positively recovered by treatments. In particular, membrane separators having fine pores, such as a ultrafiltration (UF) membrane and a reverse osmosis (RO) membrane, are widely used because they are capable of removing high-molecular-weight organic substances and obtaining treated water of high quality. On the other hand, the membrane separators have a small membrane pore size, and thus when the concentration of organic substances flowing into the separators increases, the organic substances easily accumulate on a membrane surface to significantly increase filtration resistance, thereby causing difficulty in water flow. In such a case, it is effective for a stable treatment to install a biological treatment system in a stage prior to membrane separator in order to reduce the concentration of organic substances in wastewater before a membrane separation treatment.
When a biological treatment is performed in a stage prior to a membrane separation treatment, for example, as shown in FIG. 3, organic-containing wastewater is biologically treated in an aeration tank 21, the biologically treated water is successively subjected to coagulation treatments in coagulation tanks 22 and 23 by adding a coagulant thereto, and the water undergoing coagulation treatments is subjected to solid-liquid separation in a sedimentation tank 24, followed by filtration of the separated water with a filter 30 and then membrane separation of the filtered water with a membrane separator 40. Such an apparatus for treating organic-containing wastewater is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2005-238152.
However, even if ready biodegradable organic substances are treated in the biological treatment step prior to the membrane separation treatment, several % of metabolites of refractory microorganisms is produced in the process of degrading organic substances in wastewater and is contained in biologically treated water. The metabolites are considered to be produced in a process in which the microorganisms mainly produced by degradation of organic substances contained in raw water are degraded by food chains between the microorganisms, many of the metabolites have a relatively high molecular weight and thus cause clogging of a membrane surface of the membrane separator when the concentration increases. Therefore, even when the biological treatment system is installed in a stage prior to the membrane separator, the concentration of the biological metabolites produced increases as the concentration of the organic substances in wastewater increases, and thus the membrane separator tends to become gradually difficult to stably operate.
Also, even when organic substances are removed by an ion exchange resin or oxidation treatment as an advanced treatment for recovering water, an increase in the concentration of organic substances in the water supplied to the advanced treatment step results in an increase in amount of the ion exchange resin used, an increase in exchange frequency of the ion exchange resin, or an increase in amount of the oxidizer used, causing a factor which inhibits stabilization of the treatment.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-238152